Systems

FIG. 1, Resistance.

Anyone who has held a reasonably heavy wheel, like a bicycle wheel, knows that it feels strange to try to move it around when it's spinning. In fact, a spinning wheel resists rotation at right angles to its spin-axis, as shown in Fig.1, with the spin-axis in green, and the resistance, or input-axis, in red.

A gyroscope is a wheel mounted in a series of bearings, or gimbals, that allow complete freedom of rotation in all directions. In theory, whichever way the mounting tilts, the spin-axis will continue to point in its original direction, as shown in Fig.2.

However, if the resistance is overcome with enough force, an equal and opposite reaction will be felt in a third axis, perpendicular to both the spin-axis and the input-axis, shown in Fig.3, in

FIG. 3, Precession.

orange. This is known as the output axis. For example: imagine holding the axle of the bicycle wheel in both hands, the wheel being vertical. As you turn it left or right, you will feel a powerful torque twisting it, trying to tilt the wheel from vertical to horizontal.

Of course, in the real world, no bearing can be completely frictionless, therefore a gyroscope’s gimbals can not be completely free. Their drag acts in the same way as turning the bicycle wheel, and the gyroscope reacts by drifting slowly from its original orientation. This drift is known as torque-induced precession. It might appear to be an unwanted side effect, but in fact it was harnessed, and used to create the gyrocompass.